1. Field of the Invention
The present invention relates to a vehicle motion control apparatus that performs an automatic pressurization control for generating a wheel cylinder hydraulic pressure in order to control a vehicle motion independently of a brake operation by a driver by utilizing an accumulator hydraulic pressure.
2. Description of the Related Art
Conventionally, there has been widely known a brake apparatus provided with a hydraulic pump, a motor for driving the hydraulic pump, an accumulator that stores brake fluid whose pressure is increased by the drive of the hydraulic pump by the motor, and a hydraulic booster that assists the brake pedal operation by a driver by utilizing the pressure of the brake fluid stored in the accumulator (hereinafter referred to as “accumulator hydraulic pressure”) (see, for example, Japanese Unexamined Patent Application No. 2004-066941).
The apparatus disclosed in the aforesaid reference is configured to drive the motor (accordingly, hydraulic pump) when the accumulator hydraulic pressure becomes less than a predetermined lower limit value, and to stop the hydraulic pump when the accumulator hydraulic pressure exceeds a predetermined upper limit value that is greater than the lower limit value. Accordingly, the accumulator hydraulic pressure is adjusted in principle to “the pressure (high pressure) within the range between the lower limit value and the upper limit value” that is greater than a lower limit value of the accumulator hydraulic pressure (hereinafter referred to as “assist limit value”) necessary for sufficiently assisting the brake pedal operation by the hydraulic booster.
Further, the apparatus disclosed in the aforesaid reference is provided with plural solenoid valves for adjusting the brake hydraulic pressure in the wheel cylinder (hereinafter referred to as “wheel cylinder hydraulic pressure”). With this structure, the apparatus disclosed in the aforesaid reference can execute a known anti-skid control (hereinafter referred to as “ABS control”) by controlling the plural solenoid valves, and additionally, can execute an automatic pressurization control (e.g., an over-steer suppression control, or the like) that generates a wheel cylinder hydraulic pressure for controlling the vehicle motion independently of the brake operation by a driver by controlling the plural solenoid valves with the use of the accumulator hydraulic pressure adjusted to the high pressure.
Meanwhile, in case where the ABS control is executed, or in case where the operation in which the ON-OFF operation of the brake pedal is repeatedly performed (hereinafter referred to as “pumping brake operation”) is executed, for example, the brake fluid in the brake hydraulic circuit is returned to the reservoir. Accordingly, the accumulator hydraulic pressure maintained to be the high pressure is in most cases reduced to become less than the lower limit value. In this case, the hydraulic pump is started to be driven, so that, in general, the accumulator hydraulic pressure immediately increases after that and is returned to be the value not less than the lower limit value.
However, in case where the degree of the increase or decrease in the wheel cylinder hydraulic pressure in the ABS control is extremely great and its cycle is short, or in case where the cycle of ON-OFF of the brake operation in the pumping brake operation is extremely short, the average speed of the brake fluid in the brake hydraulic circuit returned to the reservoir (hereinafter referred to as “consumption speed of brake fluid”) becomes extremely fast. When the consumption speed of the brake fluid is extremely fast as described above, the accumulator hydraulic pressure still reduces even if the hydraulic pump is continuously driven, and as a result, there may occur the phenomena in which the accumulator hydraulic pressure reduces to the value near the assist limit value.
When the automatic pressurization control is started with the accumulator hydraulic pressure reduced to the value near the assist limit value, there may be the case where the accumulator hydraulic pressure still reduces even if the hydraulic pump is continuously driven, since the brake fluid is supplied to the wheel cylinder in order to increase the wheel cylinder hydraulic pressure. As a result, the accumulator hydraulic pressure can be less than the assist limit value.